1. Field of the Invention
This invention relates to compounds, to processes of their preparation, to pharmaceutical compositions containing them and to their medical use as agonists at kappa opioid receptors.
The present invention also relates to compositions and method for the treatment and/or prevention of itch, also known as pruritus, which has many causes. The compositions, which are formulated for topical and systemic administration, contain kappa opiate receptor agonists that are substantially devoid of central nervous system effects, and, thus, have very little, if any potential for producing side effects associated with centrally acting kappa opiate receptor agonists.
2. Reported Developments
A) Kappa (κ)-Receptor Agonists as Analgesics
Opium and its derivatives are potent analgesics that also have other pharmacological effects, and exert their effects by interacting with high-affinity receptors.
It has been shown by investigators that there are at least three major opioid receptor types in the central nervous system (hereinafter CNS) and in the periphery. These receptors, known as mu (μ), delta (δ) and kappa (κ), have distinct pharmacological profiles, anatomical distributions and functions. [See, for example: Wood, P. L., Neuropharmacology. 21, 487-497, 1982; Simon, E., J. Med. Res. Rev., 11, 357-374, 1991; Lutz et al., J. Recept. Res. 12, 267-286; and Mansour et al., Opioid I. ed. Herz, A. (Springer, Berlin) pp. 79-106, 1993.] The δ receptors are abundant in CNS and mediate analgesia, gastrointestinal motility and various hormonal functions. The μ receptors bind morphine-like drugs and mediate the opiate phenomena associated with morphine, including analgesia, opiate dependence, cardiovascular and respiratory functions, and several neuroendocrine effects.
The κ receptors have a wide distribution in CNS and mediate a spectrum of functions including the modulation of drinking, water balance, food intake, gut motility, temperature control and various endocrine functions. They also produce analgesia. [See, for example: Leander et al. J. Pharmacol. Exp. Ther. 234, 463-469, 1985; Morley et al. Peptides 4, 797-800, 1983; Manzanares et al., Neuroendocrinology 52, 200-205, 1990; and Iyengar et al. J. Pharmacol, Exp. Ther. 238, 429-436, 1986.]
Most clinically used opioid analgesics such as morphine and codeine act as μ receptor agonists. These opioids have well-known, undesirable and potentially dangerous dependence forming side effects. Compounds which are κ-receptor agonists act as analgesics through interaction with κ opioid receptors. The advantage of these agonists over the classical μ receptor agonists, such as morphine, lies in their ability to cause analgesia while being devoid of morphine-like behavioral effects and addiction liability.
A large number of classes of compounds which act as agonists at κ opioid receptors have been described in the art including the following illustrative classes of compounds.
U.S. Pat. No. 4,065,573 discloses 4-amino-4-phenylcyclohexane ketal compounds having analgesic activity.
U.S. Pat. No. 4,212,878 discloses phenylacetamide derivatives having analgesic properties and reduced physical dependence liability properties, relative to morphine and methadone.
U.S. Pat. No. 4,145,435 discloses N-(2-amino-cycloaliphatic)-phenylacetamide compounds having analgesic activity and narcotic antagonist activity.
U.S. Pat. No. 4,098,904 discloses N-(2-amino-cycloaliphatic)-benzoamides and naphthamides useful for relieving pain.
U.S. Pat. No. 4,359,476 discloses substituted cycloalkane-amides useful as analgesics and having low abuse liability.
U.S. Pat. No. 4,438,130 discloses 1-oxa-, aza- and thia-spirocyclic compounds having analgesic activity, low physical dependence and abuse liability properties and little dysphoric inducing properties.
U.S. Pat. No. 4,663,343 discloses substituted naphthalenyloxy-1,2-diaminocyclohexyl amides as analgesics.
U.S. Pat. No. 4,906,655 discloses 1,2-cyclohexylaminoaryl amides having high kappa-opioid affinity, selectivity and potency and useful as analgesics, diuretics, anti-inflammatory and psychotherapeutic agents.
B) Kappa (κ)-Receptor Agonists as Anti-Pruritic Agents
The prior art has investigated the physiology and treatment of pruritus as illustrated hereunder.
Itch is a well known sensory state associated with the desire to scratch. As with pain, itch can be produced by a variety of chemical, mechanical, thermal or electrical stimuli. In addition to the difference in the sensory quality of itch and pain, they also differ in that (1) itch, unlike pain, can only be evoked from the superficial layers of skin, mucosa, and conjunctiva, and (2) itch and pain usually do not occur simultaneously from the same skin region; in fact, mildly painful stimuli, such as scratching, are effective in eliminating itch. In addition, the application of histamine to skin produces itch but not pain. Itch and pain are further dissociated pharmacologically: itch appears to be insensitive to opiate and non-steroidal anti-inflammatory drug (NSAID) treatment, both of which are effective in treating pain.
Although itch and pain are of a class in that both are modalities of nociception transmitted by small unmyelinated C fibers, evidence that itch is not just a variety of low-threshold pain is overwhelming. Itch leads to the reflex or urge to scratch; pain leads to withdrawal. Itch occurs only in the skin; pain arises from deeper structures as well. Heat may stop pain but usually increases pain. Removal of the epidermis eliminates itch but causes pain. Analgesics, particularly opioids, relieve pain but often cause itch (see, for example J. Am. Acad Derm. 24: 309-310, 1991). There can be no doubt that itching is of eminent clinical importance; many systemic and skin diseases are accompanied by persistent or recurrent itch attacks. Current knowledge suggests that itch has several features in common with pain but exhibits intriguing differences as well (see, for example, W. Magerl, IASP Newsletter, pp. 4-7, September/October 1996).
McMahon et al. (TINS, Vol. 15, No. 12, pp. 497-501, 1992) provides a description of stimuli (Table a) and a comparison of the established features of itch and pain (Table b):
TABLE aStimuli that can elicit or augment itchPhysicalMechanical. Light touch, pressure, suction.Thermal. Warming.Electrical. Focal transcutaneous repetitive stimulation, transcutaneousconstant current stimulation, intraneural microstimulation.ChemicalNon-specific irritants. Acids, alkalis.Inflammatory mediators. Histamine, kallikrein, bradykinin, prostaglandins.Histamine-releasing substances. Compound 48/80, protamine C3a.Peptidases. Mucunain, papain, trypsin, mast cell chymase.Neuropeptides. Substance P, vasoactive intestinal polypeptide,neurotensin, secretin.Opioids. Morphine, β-endorphin, enkephalin analogues.
TABLE bComparison of the established features of itch and painITCHPAINPsychophysiologyTissueSkin. MucousMost tissuesmembranesStimulusSee Table aMany stimuliIntraneural microstimulationOccasionallyYesSecondary sensationsAlloknesis (itchy skin)HyperalgesiaPsychogenic modificationPronouncedPresentCounterstimuliSratching, pain, coolingTactile stimuli,coolingNeurophysiologyPrimary afferent neuronesC- and Aδ-fibresC- and Aδ-fibresFlare sizeLargeSmallSpinal pathwayAnterolateral funiculusAnterolateralfuniculusProtective reflexesSratching, sneezingFlexion,guardingAutonomic reflexesYesYesPharmacologyCapsaicin sensitivityYesChemogenicpain; yesNSAID sensitivityProbably notYesMorphine sensitivityNoYesAbbreviations: NSAID, non-steroidal anti-inflammatory drugs. 
Experimental focal itch stimuli are surrounded by a halo of seemingly unaffected tissue where light tactile stimuli are capable of eliciting itch-like sensations. The term itchy skin or alloknesis has been coined for these secondary sensations that are reminiscent of the features of secondary hyperalgesia evolving around a painful focus. A crucial observation is that itch and pain usually do not coexist in the same skin region and a mild noxious stimulus such as scratching is in fact the singly most effective way to abolish itch. This abolition of itch can be prolonged producing an ‘antipruritic state’. Although mild scratch is often not painful, microneurographic recordings from humans have directly determined that such stimuli are among the most effective ways to excite cutaneous unmyelinated nociceptive afferents. (See, for example:                Shelly, W. B. and Arthur, R. P. (1957)Arch. Dermatol. 76.296-323;        Simone, D. A. et al. (1987) Somatosens. Res. 5, 8 1-92;        Graham, D. T., Goodell, H. and Wolff, H. G. (1951) J. Clin. Invest. 30, 37-49;        Simone. D. A., Aireja, M. and LaMotte, R. H. (1991) Somatosens Mot. Res. 8, 27 1-279;        Torebjörk, E (1985) Philos. Trans. R. Soc. London Ser. B 308, 227-234; and        Vailbo. A. B. Hagbarth, K. E., Torebjörk, H. E. and Wallin, B. G. (1979) Physiol. Rev. 59, 919-957).        
Physiologically, there is evidence that substance P released from nociceptor terminals can cause the release of histamine from mast cells. Activation of mast cells, with release of the pruritogen histamine, occurs in immediate type hypersensitivity diseases, such as anaphylactic reactions and urticaria. Urticarial eruptions are distinctly pruritic and can involve any portion of the body, and have a variety of causes beyond hypersensitivity, including physical stimuli such as cold, solar radiation, exercise and mechanical irritation. Other causes of pruritus include: chiggers, the larval form of which secretes substance that creates a red papule that itches intensely; secondary hyperparathyroidism associated with chronic renal failure; cutaneous larva migrans, caused by burrowing larvae of animal hookworms; dermal myiasis, caused by maggots of the horse botfly, which can afflict horseback riders; onchocerciasis (“river blindness”) caused by filarial nematodes; pediculosis, caused by lice infestations; enterobiasis (pinworm) infestations, which afflict about 40 million Americans, particularly school children; schistosome dermatitis (swimmer's itch); and asteatotic eczema (“winter itch”). The role of histamine or other endogenous pruritogens in mediating itch associated with these and other pruritic conditions, such as atopic dermatitis, it is not yet well established. For atopic dermatitis, in particular, it appears that itch is not inhibited by antthistamines, but by cyclosporin A, a drug which inhibits the production of cytokines which have been proposed as potential pruritogens
Current therapies for the treatment of itch include a variety of topical and systemic agents, such as steroids, antihistamines, and some psychotherapeutic tricyclic compounds, such as doxepin hydrochloride. Many such agents are listed in PDR Generics (see Second Edition, 1996, p. cv for a listing of said agents). The limitations of these agents are well known to medical practitioners, and are summarized in the “Warnings” and “Precautions” sections for the individual agents listed in PDR Generics. In particular, the lack of complete efficacy of antihistamines is well known, but antihistamines are frequently used in dermatology to treat pruritus due to urticaria, atopic dermatitis, contact dermatitis, psoriasis, and a variety of other conditions. Although sedation has been a frequent side effect of conventional systemically administered antihistamines, a new generation of antihistamines have been developed that are nonsedating, apparently due to their inability to cross the blood-brain barrier.
Intravenous administration of opiate analgesics, such as morphine and hydromorphone has been associated with pruritus, urticaria, other skin rashes, wheal and flare over the vein being injected. These itch and itch-related reactions are believed to be due to a histamine-releasing property of these opiates, via mast cell degranulation. These opiates are thought to act upon the mu subtype of opiate receptor, but the possibility of interactions at the other principal opiate receptor subtypes (delta and kappa) cannot be excluded since these and other pruritogenic analgesics are not pure mu agonists. The cellular loci of the receptor type(s) mediating the itching effect is not known, although the mast cell is a possible candidate since opiates cause histamine release from these cells. However, some investigators have suggested that the frequent inability of antihistamines to block morphine-induced itching suggests a non-histaminergic mediation of opiate-induced itching—a mechanism which could involve central opiate receptors. Although i.v. morphine only occasionally results in generalized itching (in about 1% of patients), pruritus is more prevalent in opiate analgesia with epidural (8.5%) or intraspinal (45.8%) administration. (See, for example: Bernstein et al., “Antipruritic Effect of an Opiate Antagonist, Naloxone Hydrochloride”, The Journal of Investigative Dermatology, 78: 82-83, 1982; and Ballantyne et al., “Itching after epidural and spinal opiates”, Pain, 33: 149-160, 1988.)
To date, treatment with opiates has not only proven useless in the treatment of itch, but appears to exacerbate itch in man. The consistent findings from human studies indicate that whether by central or peripheral mechanisms, opiates appear to promote rather than prevent itching, and that opiate antagonists have anti-pruritic activity.
Human clinical studies have generally shown that opiates cause itching and there is evidence that these effects can be reproduced in animal models, where itching sensations per se cannot be reported, but scratching behavior can be observed. (See, for example: Thomas et al., “Microinjection of morphine into the rat medullary dorsal horn produces a dose-dependent increase in facial-scratching”, Brain Research, 195: 267-270, 1996; Thomas et al., “Effects of central administration of opioids on facial scratching in monkeys”, Brain Res., 585: 315-317, 1992; and Thomas et al., “The medullary dorsal horn: A site of action of opioids in producing facial scratching in monkeys”, Anesthesiology, 79: 548-554, 1993).
We have now surprisingly discovered that kappa agonist compounds, which are substantially devoid of central nervous system effects, in pharmaceutically acceptable vehicles for systemic and topical formulations possess anti-pruritic activity in addition to anti-hyperalgesic activity.